Optical fiber, as well-known in the art, is broadly classified into three types, the oldest and least expensive is a step-index fiber made from a higher index rod of glass surrounded by a lower index glass coating, the second oldest and most costly is a graded-index mode fiber having a core made from many layers of glass wherein the core will have an inconsistently distributed refractive index, and the newest is an optical fiber having a core of a uniform refractive index known as a single-mode fiber. The single-mode fiber is actually a step-index fiber which was pulled to a very thin core diameter. The step-index and graded-index fibers carry thousands of optical modes and are called multimode (MM) fibers, and are mainly used for a middle-distance communication or a short-distance communication. The single-mode fiber carries only the lowest order mode and is used for long-distance communication.
To produce single mode or multi-mode fiber preform, there are three methods, a outside vapor deposition (OVD), ie. outside vapor phase oxidation (OVPO), a vapor phase axial to deposition (VAD) and a modified chemical vapor deposition (MCVD). The MCVD method has some advantages over the others, wherein few impurity materials are introduced because the method is performed in a sealed tube, and a precise distribution of refractive index can be easily controlled because very thin glass films are deposited with each other. Accordingly, of the above described methods, the MCVD method is used more than 60% of the time.